Method of manufacture of article with self-locking screw thread



Jan. 7, 1969 E. G. GABBEY 3,419,928

METHOD OF MANUFACTURE OF ARTICLE WITH ELF- CKING SCREW THREAD -gil?,Filed April 29, 1965 United States Patent O 3,419,928 METHOD OFMANUFACTURE OF ARTICLE WITH SELF-LOCKING SCREW THREAD Eric G. Gabbey, PO. Box 43271, Los Angeles, Calif. 90043 Original application Apr. 29,1965, Ser. No. 451,784, now

Patent No. 3,358,726, dated Dec. 5, 1967. Divided and this applicationApr. 17, 1967, Ser. No. 631,445. U.S. Cl. -10 4 Claims Int. Cl. F16b39/28 ABSTRACT OF THE DISCLOSURE Typically, a bolt has a small boreparallel to but offset from the bolt axis to form a thin, deformablewall at the base of the thread at one side of the Ibolt. The thread isthen expanded locally to increase the pitch diameter at the thin wall,providing additional holding power to resist vibration. The inventionmay :also be applied to a nut having an internal thread.

This application is a division of my copending patent application Ser.No. 451,784, led Apr. 29, 1965, for Article With Self-Locking ScrewThread and Method of Manufacture Thereof, now Patent No. 3,358,726.

The present invention relates generally to screw threaded articles suchas nuts, bolts, and other types of fasteners, and more especially tosuch articles having a novel type of self-locking thread and to a methodof making such a thread.

It has been discovered that under continued vibration, such as isencountered in airplanes, plain nuts and bolts `become loosened by thevibration and, when the initial tension under which they are placed isreleased, they rapidly lose their holding power. One means used toresist this loosening under vibration is to provide a lock washer ofsome type or a gripping configuration on a bolt head.

However, this has proved unsatisfactory under many circumstances forvarious reasons.

In order to meet this problem of resisting vibration, it has also becomewell known to add friction devices to the nut or bolt, this class offasteners commonly being referred to as prevailing-torque typefasteners. These take such formations as an insert in the bolt or anelastic ring on the nut, such elements generally being made fromnonmetallic materials, such as the synthetic resins or plastics. Oneserious limitation on the use of such devices is that the nonmetallicmaterials generally do not withstand high temperatures and,consequently, fail in their objective at elevated temperatures.

An ideal fastener having helical screw threads on it and designed toresist vibration should ihave several characteristics to the maximumIpossible degree. For example, it should resist vibration uniformly overa wide range of axial travel so that it does not tend to spin freely andloosen very rapidly after it has backed off slightly under the influenceof vibration. It should also be reusable so that the same fastener canbe removed and replaced a number of times without the necessity of usingnew ones each time the fastener is loosened.

Among other desirable characteristics of the ideal fastener are that itshould be adapted to any manufacturers product, therefore making itwidely available for use; that it should comprise only a single piece sothat there is no washer or separate member to `become lost or requirereplacement; and also that it should be comparatively inexpensive tomanufacture.

It is also highly desirable that a vibration-resistant fastener of thistype be all metal, so that full advantage can be taken of modern hightensile strength alloys and that the fastener can resist, without anydecrease in holding power, relatively high temperatures, for example inthe area of 1,000" F. or even higher. These temperatures are now beingencountered in actual practice in airplanes and missiles which ily atspeeds greater than sonic speeds so that severe vibration is encounteredalong with relatively high temperatures.

Thus, it becomes a general object of the present invention to provide avibration-resistant fastener having all of the `above advantages in themaximum possible degree and yet which is simple, uncomplicated, andinexpensive to manufacture.

It is a more specific object of the present invention to provide avibration-resistant fastener of this character which comprises anall-metal structure adapted to openate successfully under comparativelyhigh rambient temperatures.

It is also an object of the present invention to provide a novel designfor a vibration-resistant article having a helical screw thread in whichthe vibration-resistant features may be applied to either an internal orexternal thread.

The objects of the present invention are achieved by providing afastener or similar article with a self-locking screw thread comprisinga 'body of metal or other elastic material having a helical screw threadcut on the body, a portion of the screw thread being radially deformedso that it is noncircular, i.e., not of uniform radius, with respect tothe axis of the remainder of the thread. Preferably, this zone ofdeformation is spaced from one end of the screw threads so that there iseasy starting engagement at that end with mating threads but thevibrationresistant feature then comes into operation after the matinghreads have been engaged to -a predetermined extent.

An article of this character is made by drilling a hole in the body ofthe threaded article adjacent to but eccentric with respect to thethreads and then enlarging the diameter of the hole for at least aportion of its length in order to deform the screw threads locally toproduce a section thereof that has the noncircular characteristicmentioned. The drilled hole is placed close enough to the screw threadsthat, by inserting a tool in the hole, the or slightly deformed. But,being elastic, this deformed wall between the hole and the threads canbe stretched portion can, if necessary, be returned partway toward itsoriginal shape by engagement of the threads with a mating thread, yetstill maintain a firm, vibration-resistant frictional engagement withthe mating threads.

How the above objects and advantages of the invention, as well as othersnot specifically mentioned herein, are attained will be readilyunderstood by reference to the following description and the annexeddrawing, in which:

FIGURE l is a combined side elevation and longitudinal median sectionthrough a bolt having an external thread, showing one stage in theformation of the vibration-resistant construction;

FIGURE 2 is an end elevation of the bolt of FIG- URE l;

FIGURE 3 is a view similar to FIGURE l showing a later stage in thedevelopment of the vibration-resistant construction;

FIGURE 4 is an end elevation of the completed thread;

FIGURES 5 and 6 are enlarged fragmentary sections corresponding,respectively, to FIGURES l and 3;

FIGURE 7 is a combined side elevation and longitudinal section through anut having an internal thread, illustrating the application thereto ofthe invention, one stage in the manufacture of the vibration-resistantconstruction being illustrated;

FIGURE 8 is a fragmentary section through the nut of FIGURE 7, showing alater stage in the development of the vibration-resistant structure; and

FIGURE 9 is a fragmentary section through the nut of FIGURE 8 on line 99 thereof.

Referring now to the drawing, and more particularly to FIGURES l and 2,there is shown therein at an intermediate stage in the manufacture atypical screw-threaded bolt indicated generally at 10, having acylindrical shank 11 on the outside of which are cut helical threads 12which are here shown as being a standard V-thread, but it will berealized that the shape of the thread is in no way limitative upon theinvention.

The structure of the completed article will be most readily understoodby describing its manufacture. The rst step is to drill in shank 11 ahole 14 which is eccentric with respect to the threads 12, the axis 15of hole 14 being spaced from and parallel to the axis 16 of threads 12.Hole 14 is drilled from the end of the shank and is a blind hole,extending for any desired portion of threads 12. After the hole 14 iscompleted, it is counterbored at the outer end 18 to provide a sectionof slightly larger diameter. The completed hole is shown in greaterdetail in FIGURE 5, wherein the diameter of the original hole is D1 andthe diameter of the counterbore is D2, D2 being slightly larger than D1by an amount which will be determined by the material used, the diameterof the bolt, and other considerations.

Generally speaking, D1 is made as small as is practical, in order toretain as much of the metal in the shank of the bolt as possible andthereby to retain the full strength of the bolt by minimizing thereduction in cross section. The spatial relationship between the axis ofthe drilled hole and the axis of the threads is selected so that thereis between bore 14 and the roots of threads 12 a relatively thin wallindicated at 20 in FIGURE 5. The thickness of this wall at its minimumpoint will be determined by the designer according to the strength andother physical properties of the metal in bolt shank 11.

After hole 14 is counterbored as shown in FIGURE 5, a suitable expansiontool is inserted in the hole, the leading end of the tool being rounded.For practical reasons, the tool preferred is a shank 22 which pushes around steel ball 23 to the bottom of hole 14; but the invention is notso limited and the shank 22 may have aixed to it a suitably shaped endportion. While it is preferred that the expanding portion of the tool isspherical, a tapered or conical end may be used on the tool instead, ifdesired. The diameter D3 of the ball 23, or the equivalent dimension ofthe tool in a plane transverse to the axis of bore 14, is madesubstantially equal to or slightly less than D2 in order that the toolpasses freely through the counterbore 18 to the shoulder where thesmaller diameter D1 commences. However, the ball diameter D3 is greaterthan D1 and, consequently, the tool must be forced into the reducedportion of the bore 14 by the application of substantial pressureaxially of shank 22.

For purposes of disclosure, the ball 23 will be considered as theoperating portion of the tool. As ball 23 is forced into lbore 14, theball 23 acts to enlarge the diameter of the hole 14 to equal thediameter of the ball. It will be realized that the mass of metalradially inward from bore 14 is comparatively large and resistsdeformation, whereas the thickness of the wall at 20 is much smaller andis designed to yield to enlarge bore 14. Accordingly, as the tool ismoved axially of bore 14, the wall at 20 is deformed outwardly, as shownenlarged in FIGURE 6, by stretching the metal in the wall since it it atthis portion of the article that the resistance to enlargement of bore14 is the least. The final shape of bore 14 is a hole of approximatelyuniform diameter throughout its length but with the inner end offsetlaterally from the outer counterbore 15. At the end of the operation,shank 22 is withdrawn, leaving ball 23 in place. This 4 procedureeliminates any problems attendant on removing the ball.

The metal at wall 20 is now stretched beyond its elastic limit and soretains a permanent deformation. This deformation appears externally ofthe threaded .shank as shown greatly exaggerated in FIGURES 3, 4 and 6,in which there is a zone 24 wherein the radius of the threads 12 hasbeen enlarged locally. This enlargement is localized as seen clearly inFIGURE 4, and the threads at this point may be described as beingnoncircular with respect to axis 16, the axis of the remainder of thethreads, since for a portion of each lfull turn the radius of eachthread has been increased and, therefore, is not uniform Ifor 360 aroundaxis 16.

In ordinary circumstances, zone 24 is spaced `from both ends of thethreads 12, that is, from the ends of the threaded portion of shank 11.The spacing with respect to the end of the threads adjacent the head ofthe bolt is of no particular consequence and may be varied as desired,according to the length of the bolt. However, with respect to theopposite end of the bolt, which is the end that iirst enters the nut ormating thread on the .part to which the bolt is attached, it ispreferred that the deformed zone 24 be spaced from the end of thebolt bya distance 25 which is equal to about two full threads, or morespecifically, the distance 25 equals about twice the lead of threads 12.Of course, a greater or smaller distance may be used if desired, thelength being subject to various considerations.

This spacing of the deformed thread zone from the end of the bolt leavesabout two full normal threads which permit easy normal startingengagement between the bolt and the nut or other article into which itis to be threaded. These two normal 'full threads allow the bolt to bestarted easily in a nut and to develop a good grip with mating threadson the nut before the deformed zone 24 enters the mating threads.

In order to make a full 4disclosure of a typical embodiment of theinvention, certain exemplary dimensions will be given; but it is to beunderstood that these are merely typical and in on way limitative uponthe invention, since all dimensions are subject to determination andselection based upon strength of the material used,

its elastic limit, the diameter of the bolt, and various other physicalcharacteristics. However, lfor a quarterinch standard bolt, thethickness t of wall 20 at the base of threads 12 may be typically .010in., more or less. The diameter of bore 14 and the location of axis 15are then selected so that the maximum increase in the radius of thethreads as indicated at 26 in FIGIURE 6 is approximately .005 inch, moreor less. To accomplish this requires that the difference between D1 andD3 be approximately .005 inch.

The axial length of the deformation zone 24 is typically o-f the orderof four or ive threads but again this is subject to variation accordingto the judgment of the designer.

As the bolt constructed as described is threaded into a nut or otherarticle, it will be appreciated that the enlargement of the diameter ofthe bolt threads in the zone '24 creates a tight fit between them andthe mating threads on the nut which exerts a continuous, frictional gripupon the mating threads; and that this frictional grip is uniformregardless of the angular position of the bolt as long as one or more ofits deformed threads are in engagement with a mating thread. Theexternal pressure of the nut on threads 112 of the bolt may tend toreturn the enlarged or eccentric portion of the threads in the zone 24to its original diameter. Ball 23 resists this external pressure; butelsewhere Ifor the length of bore 14 some compression in zone 24 mayoccur; but, because of the elastic nature of the metal `from which bolt10 is made, the degree of restoration is less than the originaldeformation so that the deformed threads can continue to exert africtional grip on the surrounding threads. That this is tr-ue has beendemonstrated in actual practice by screwing home a bolt constructed asdescribed, and then removing it from the nut; and repeating bothoperations several times. The friction between the bolt and thesurrounding threads may be suiciently great that it is advisable toprovide the threads with some type of dry lubricant to reduce thisfriction. However, it has been found in actual practice that the boltconstructed as described can be reused a number of times, a suficientnumber of times to meet the requirements of military specifications.

The elasticity of wall 20 also enables the bolt to hold firmly in matingthreads within the 4full range of dimensions or normal manufacturingtolerances.

The invention has so far been described as applied to an article, forexample a bolt, having an external thread; but the invention may beequally well applied to an article, -for example a nut, having aninternal thread. Such an article is illustrated in FIGURES 7, 8 and 9,in which the nut 30 is provided with an internal helical thread 12ahaving a central axis at 16a. The bore 14a and the counterbore 15a aremade and located in the manner previously described and are subsequentlyenlarged by ball 23. The resultant enlargement of the bore deformsthreads 12a over a zone 24a.

'Ihe only diierence between applying the invention to an internal threadand to an external thread is that the radius of the threads is decreasedas at 26a in the deformation zone 24a, rather than enlarged aspreviously described; but in each instance the threads may be describedas being locally deformed to be no longer exactly circular or of uniformradius in the zone of deformation. Again, the zone of deformation isspaced from the ends of the threaded portion of the article, preferablybeing spaced by a distance equal to about twice the lead of the threadsfrom the portion of the threads initially engaged with a mating thread,to provide for =free starting.

From the yforegoing description, it will be understood that variouschanges and modifications in the detailed construction of a self-lockingthread and the manner of making it may occur to persons skilled in theart without departing from the spirit and scope of the presentinvention. Accordingly, it is to be understood that the abovedescription is considered to be illustrative of, rather than limitativeupon, the invention as defined by the appended claims.

I claim:

1. The method of making an article having self-locking screw threadsthat includes the steps of drilling a hole in the article adjacent buteccentric relative to the threads,

counterboring the hole at one end thereof to receive a tool,

5 and expanding the diameter of the inner portion of the hole tosubstantially the diameter of the tool but eccentric relative "to thecounterbore to alter the radius of the screw threads locally withrespect to the original axis of the threads.

2. The method according to claim 1 in which the original axis of thebore and counterbore is substantially parallel to the original axis ofthe threads.

3. The method of making a self-locking thread on a metal article thatincludes the steps of drilling a hole in the article adjacent thethreads thereon, the axes of the .hole and the threads being parallelbut spaced apart, counterboring the hole at one end, .and inserting atool in the hole beyond the counterbore and stretching the metal betweenthe hole and the threads beyond the elastic limit to deform the threadslocally,

said deformation of the threads being in a zone spaced from both ends ofthe threads. 25 4. The method of making a self-locking thread on a metalarticle that includes the steps of drilling a hole in the articleadjacent the threads thereon, the axes of the -hole and the threads beinparallel but spaced apart, counterboring the hole at one end, insertingin the hole a ball having a diameter no greater than the diameter of thecounterbore and stretching the metal between the hole and the threadsbeyond the elastic limit to alter the radius of the threads locally, andleaving said ball in said hole.

References Cited UNITED STATES PATENTS 2,856,617 10/1958 Widmann 10-10FOREIGN PATENTS 8,475 6/ 1887 Great Britain.

45 LEONIDAS VLACHOS, Primary Examiner.

U.S. Cl. X.R. 10-27, 86; 151.-22 i y

